1. Field of the Invention
The present invention relates to an endoscope capable of simultaneously observing a forward field of view and sideward field of view.
2. Description of the Related Art
In recent years, endoscopes have been widely used in the medical field and the industrial field. The endoscope enables observation of an inside of a subject with an elongated insertion portion inserted into the subject. A variety of types of such endoscopes have been proposed and put to practical use, such as a direct-viewing type endoscope in which an observation lens and an illumination lens are provided on a distal end surface of a distal end portion provided on a distal end side of an insertion portion, and a side-viewing type endoscope in which an observation lens and an illumination lens are provided on a part of a side surface of a distal end portion of an insertion portion.
Further, in recent years, various endoscopes have been proposed, in Japanese Patent No. 4955838, for example, and put to practical use, which are configured to be capable of simultaneously observing not only a forward field of view with respect to a distal end portion of an insertion portion, but also a field of view in a circumferential direction positioned sideward along a circumference of an outer circumferential side surface of a distal end portion in order to expand the observation range.
This type of endoscope is configured to be provided with, for example, a forward-observing objective lens that forms an image of the forward field of view and a sideward-observing objective lens which is provided behind the forward-observing objective lens and forms an image of the sideward field of view to allow a wide field of view to be observed. In this case, some sideward-observing objective lenses are configured to serve also as forward-observing objective lenses.
This configuration enables simultaneous observation of a sideward field of view in a circumferential direction in addition to a forward field of view with respect to a distal end portion of an insertion portion of an endoscope.
Japanese Patent Application Laid-Open Publication No. 2003-319903 and Japanese Patent Application Laid-Open Publication No. 2010-194191, for example, disclose illuminating means for the conventional types of endoscopes, configured to provide a ring-shaped optical system member so as to surround a circumferential edge of a forward-observing lens placed on a distal end surface of an insertion portion and cause a light flux from a light guide fiber to be incident to the ring-shaped optical system member.